Wireless communications systems may be generally categorised into frequency division duplexing (FDD) systems and time division duplexing (TDD) systems. In FDD systems, the transmitter and receiver typically operate simultaneously but at different frequencies, and the duplexer provides isolation between them. Due to the finite duplexer transmitter-receiver isolation, out-of-band noise present in the transmitter is especially critical, as it may leak into the receiver band and irreparably corrupt the sensitivity of the receiver. Far-out-of-band noise is an important concern in transmitting devices, especially when they coexist with, for example, Global Positioning System (GPS), Wireless Local Area Network (WLAN) and/or Worldwide Interoperability for Microwave Access (WiMAX) components on the same smartphone.
Typically, an inter-stage Surface Acoustic Wave (SAW) filter is added to suppress this transmitter leakage. In particular, an inter-stage SAW filter may be included for every band between Pre-Power Amplifier (PPA) and Power Amplifier (PA). However, this severely increases the cost of materials required and the footprint of the overall device.
When Carrier-to-Noise (CNR) values at different receiver frequency offset are lower than, for example, −160 dBc/Hz at the PPA output (which may be an example of a worst-case value), it is possible to remove the inter-stage SAW. A conventional voltage-sampling mixing transmitter is described in an article by M. Ingels, V. Giannini, J. Borremans, et al., entitled “A 5 mm2 40 nm LP CMOS 0.1-to-3 GHz Multistandard Transceiver”, ISSCC Dig. Tech. Papers, pp. 458-459, February 2010. As taught by Ingels et al., an active filter is needed to remove the Digital-to-Analogue Conversion (DAC) aliases to reconstruct the wanted signal. Furthermore, a passive pole is still needed to further reduce the out-of-band noise before a voltage-sampling mixer up-converts the signal to radio frequency (RF).
However, by introducing such passive pole, several issues will arise. First, the passive mixing operation will suffer increased gain losses, resulting in a reduction of P1 dB over the RF frequency. Second, because the passive pole is a fixed passive pole, there are no trade-offs available in the case that different receiver FDD frequency offsets are to be targeted. Third, as the passive pole introduces noise by itself too, different trade-offs might be needed when the target channel bandwidths are different.
In “A Low-Power, Low-EVM, SAW-Less WCDMA Transmitter Using Direct Quadrature Voltage Modulation” by Xin He and Jan van Sinderen, IEEE J. Solid-State Circuits, Vol. 44, No. 12, December 2009, a transmitter is described that uses direct quadrature voltage modulation. The core of the modulator is a passive voltage mixer driven by a 25% duty-cycle local oscillator, which provides significantly improved performance in terms of output noise, linearity, Local Oscillator (LO) leakage, Error Vector Magnitude (EVM) and power consumption in the transmitter. Here, the requirement to accommodate standard diversities among different countries and different operation service providers is noted in that next generation mobile handsets will need to support up to 10 Wideband Code Division Multiple Access (WCDMA) bands (Bands I, II, III, IV, V, VI, VIII, IX, X and XI), 4 Global System for Mobile communications (GSM)/Enhanced Data rates for GSM Evolution (EDGE) bands (Bands GSM850, EGMS900, DCS1800 and PCS1900), as well as the emerging standard Long Term Evolution (LTE).
One of the key challenges to the elimination of SAW filters between the transmitter and the PA is to achieve a low transmitter noise floor without sacrificing power consumption. A new passive voltage mixer is described that leads to improved performance in terms of power consumption, noise floor, linearity, LO leakage and EVM. The direct quadrature voltage modulator consists of a Low Pass Filter (LPF), mixer and PA driver. The LPF attenuates the far-out noise at the proceeding Intermediate Frequency (IF) input and also suppresses the aliases from the DAC. By switching on/off four transistors through the quadrature-phased LO with the 25% duty-cycle, the filtered IF quadrature input voltages are sequentially sampled to the voltage mixer output which sees the high input impedance of the PA driver. Due to the high input impedance of the PA driver that employs a cascode structure, the linearity of the voltage mixer is not degraded.
However, due to the 25% duty-cycle LO waveforms, the slope of the rising edges and the falling edges cannot be infinite. The switch-on and switch-off time in the mixer switches cannot be zero. These effects result in a short overlapping period during the transition between just turning off one transistor and just turning on the next which degrades the linearity of the passive voltage mixer.
The out-of-band noise problem has been extensively analysed for the most popular WCDMA bands (I, II, V). Complementary metal-oxide-semiconductor (CMOS) transmitters with receiver band CNR down to −160 dBc/Hz, as described above, are typically power hungry, and as a result typically also involve the replacement of high-voltage Gilbert mixers (e.g., as described by M. Cassia et al. in “A Low-Power CMOS SAW-Less Quad Band WCDMA/HSPA/HSPA+/1X/EGPRS Transmitter”, IEEE. J. Solid-State Circuits, vol. 44, no. 7, pp 1897-1906, July 2009, and by C Jones et al. in “Direct-Conversion WCDMA Transmitter with 163 dBc/Hz Noise at 190 MHz Offset”, ISSCC Dig. Tech. Papers, pp. 336-607, February 2007).
Feedback-based notching techniques, as described by A. Mirzaei and H. Darabi in “A Low-Power WCDMA Transmitter with an Integrated Notch Filter”, IEEE J. Solid-State Circuits, vol. 43, no. 12, pp. 2868-2881, December 2008, might impact the achievable output power when wider bandwidths and low transmitter-receiver offsets are required.
A direct quadrature voltage modulator has been proposed in an article by Xin He, Jan van Sinderen and Robert Rutten, entitled “A 45 nm WCDMA Transmitter Using Direct Quadrature Voltage Modulator with High Oversampling Digital Front-End”, ISSCC Dig. of Tech. Papers, pp. 62-63, February 2010. A highly digitised multimode transmitter is described in which a Direct Quadrature Voltage Modulator (DQVM) incorporates a high speed DAC and Digital Front-End (DFE) with high oversampling ratio capable of multimode operation by adapting the sampling data rate. In the DFE, the serial I/Q baseband inputs are first converted to parallel 10-bit or 12-bit I/Q data and then are up-sampled to 32 times the input data rate before being fed into the DAC. This up-conversion technique uses the combination of an RC pole and passive mixer to achieve both linearity and low out-of-band noise. However, losses in such a mixer are proportional to the resistor value and, especially at high carrier frequencies, they shift the linearity requirements to the baseband driving stage such that large linear signal swings must be provided on a low impedance node.
In the architecture described, a simple passive low-pass filter (LPF) after the DAC is sufficient to suppress the sampling images with the help of a high oversampling ratio. The advantage of using a high oversampling ratio in the DFE rather than an active LPF after the DAC is that the power and the area can be reduced with CMOS scaling. The I/Q matching is also improved. Moreover, it can be easily adapted to different standards by using different sampling clock frequency and bypass mode settings.
The challenge of SAW-less receiver band noise becomes more acute in the LTE standard (3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (LTE): User Equipment (UE) Radio Transmission and Reception”, v. 8.6.0, June 2009) where transmitters are needed to operate in multiple FDD bands using wider channel bandwidths and higher Peak-to-Average Power Ratios (PAPR).